I have a 10 gallon filtered tank, cycled now for ~2 months, with a thriving java fern and silk plants. It has housed 6 harlequin rasboras until last night when I decided to put my male halfmoon Finley in there. Fin has been living in a 5 gallon tank.

It's been 12 hours and every now and then I stop by the tank to observe the community together. It seems as if the harle's know to stay out of Fin's way. When they get close to him he'll flare, and they'll swim to the other side. I've seen minor chasing for like 2 seconds at a time but the harle's are just way too fast for my big boy.

Fin has found refuge in a log I have in the tank. He'll watch the harle's from the holes, and as long as they're not in his general vicinity he doesn't flare.

Is Finley's flaring a sign of aggression or just defensive? What things should I keep in mind and look for when determining whether this will be a peaceful community?

It sounds like he is showing very minimal signs of aggression, which is good. A tad bit of chasing and flaring, provided no one is showing any serious signs of stress(an unusual amount of hiding, loss of color, clamping, heavy breathing), is fairly normal and to be expected....no real cause for concern just yet. Just give them some time and keep an eye on them, Things should settle down in a few days to a week as everyone gets used to one another. ;)

It sounds like he is showing very minimal signs of aggression, which is good. A tad bit of chasing and flaring, provided no one is showing any serious signs of stress(an unusual amount of hiding, loss of color, clamping, heavy breathing), is fairly normal and to be expected....no real cause for concern just yet. Just give them some time and keep an eye on them, Things should settle down in a few days to a week as everyone gets used to one another. ;)

Great! I'll def keep an eye on them! The harlequins seem to be going on as if Finley isn't there. They're constantly mating underneath the big leafy satin plants. Lol

From what I read the rasboras prefer open spaces and are 15 gallons. It could be there bright colors remind him of Bettas.

I did a lot of research before deciding on purchasing the harlequins, knowing they'll eventually share tanks with a betta. I've read that a minimum of 10g would be okay (of course bigger the better), but the whole front of the tank is open for swimming. I also have silk big leafy plants in the back of the tank because broad leaved plants grow in their natural habitat. The harle's LOVE swimming and playing through those, they even mate! They're not too colorful, I don't think - like silvery with red tones and the black triangle.

Hm, I don't know but I've had them for 2 months as a school of 6 and they haven't shown any signs of stress or illness. The black and red colors in their scales are very distinct, nothing like how they looked when I bought them at petsmart. With the information I've gathered on several threads, 6 harle's in a 10 gallon had sufficient space with a male betta, otherwise I would not have bought them. But thank you choclate, we could have just gotten our information from different places which is fine since neither of us are wrong! I definitely agree 15g would be ideal

Harlequin Rasboras are one of my favorite schooling species, VERY peaceful and great with bettas IME. Not nippy like other tetras can be and they're certainly not colorful enough to set off a betta.
Personally, I think you've made a fantastic tank mate choice, have clearly done your research, and are providing your fish with an awesome setup....what better a sign of happiness and health then good coloration and spawning behaviors? ;)

Obviously adding to the school might make space a little tight, and bigger is of course always a little better, but a 10 gallon is perfectly sufficient for a school of 6 and a betta to live happily. ;)